In its broadest sense, power quality is a set of boundaries that allows electrical systems to function in their intended manner without significant loss of performance or life. The term is used to describe electric power that drives an electrical load and the load's ability to function properly with that electric power. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power. While “power quality” is a convenient term for many, it is the quality of the voltage—rather than power or electric current—that is actually described by the term.
When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous. Depending on its duration, the overvoltage event can be permanent or transient, the latter case also being known as a voltage spike. Electronic and electrical devices are designed to operate at a certain maximum supply voltage, and considerable damage can be caused by voltage that is higher than that for which the devices are rated.
Man-made sources of spikes are usually caused by electromagnetic induction when switching on or off inductive loads (such as electric motors or electromagnets), or by switching heavy resistive AC loads when zero-crossing circuitry is not used—anywhere where a large change of current takes place. One of the purposes of electromagnetic compatibility compliance is to eliminate such sources. Several devices have been devised to implement overvoltage protection, including, but not limited to arcing horns, zener diodes, avalanche diodes, transit, trisil, spark gaps, gas filled tubes, metal oxide varistors, and SiBar thyristors.
“Undervoltage” may be defined by many values, but a standard definition of undervoltage occurs when the nominal supplied voltage drops below a specified level, for example, 90% of nominal value. A decrease in supply voltage below 90% of the nominal value may destroy the contents of memory and internal registers, as well as cause software malfunction, which may fall into an infinite software loop state without activating the reset circuit. “Overvoltage” may be defined by many values, but a standard definition of overvoltage occurs when the nominal supplied voltage rises above a specified level, for example, 110% of nominal value. In a system with a Li-ion battery, for example, over voltage may cause catastrophic failure of the battery. There are many circuits to detect an overvoltage condition. And there are many other circuits that detect an undervoltage condition.